<?php
$contents = '<a href="http://w...content-available-to-author-only...e.com/a">Works</a>
<a href="http://w...content-available-to-author-only...e.com/a/b/c">Doesnt Work</a>
<a href="http://w...content-available-to-author-only...e.com/x/y/z">Works</a>';
$regexp = "/(<a\s[^>]*href=\")[^\"]*(\"[^>]*>.*<\/a>)/siU";
return $m[1] . 'https://w...content-available-to-author-only...e.net/newlink/' . $m[2];
}, $contents);
echo $contents;
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